Various electric loads are used in vehicles. For instance, as disclosed in U.S. Pat. No. 7,100,368 (JP 2005-307957A), a motor-driven air pump and an air switching valve are used in a secondary air injection system for introducing secondary air into an exhaust air path upstream of a catalyst for purifying exhaust gas of an engine. This system includes two N-channel power MOSFETs that respectively drive a motor (load) for driving the air pump and an electromagnetic coil (load) for driving an electromagnetic valve; and one booster circuit required for using these MOSFETs as high-side switches. By sharing the booster circuit between the two MOSFETs, cost reduction is achieved.
In cases where a fault occurs in one of the two MOSFETs in gate-source short-circuiting mode in this system, the following takes place: the output current of the booster circuit that supplies gate current is increased, and the output voltage of the booster circuit extraordinarily drops. As a result, the gate voltage of the other device also drops, and the fault that occurred in one device has an influence on the other device. This is also true in cases where one booster circuit is shared among three or more switching devices.
The above system is so constructed that the following operation is performed: the drain-source voltage of each of the two MOSFETs is monitored with an output monitor circuit to detect short-circuit failure or break failure in a load; when a fault is detected, the gate is turned off to forcibly stop energization. In cases where there is no fault in loads, a state in which an on-drive signal is supplied but the drain-source voltage does not drop is monitored. Thus, gate-source short-circuit failure can be detected using the above output monitor circuit.
However, in cases where break failure has occurred or is about to occur in a load, the drain-source voltage is constantly low regardless of the level of a driving signal. It is difficult to discriminate this from a normal on-state. Thus, the gate-source short-circuit failure (short-circuit failure at a control terminal) cannot be detected with the above output monitor circuit. As a result, a fault will have an influence on another device.
In JP 2004-215415A, in a circuit in which a plurality of power semiconductor elements is connected in parallel, a gate resistor is forcibly cut off by heat even when one or more gates of the power semiconductor elements is short-circuited due to aging or deterioration. Thus, other semiconductor elements are protected from being influenced and can continue to operate. However, the gate resistor need be formed of easily fusible material. Such a gate resistor is likely to be cut off by heat even with various noises that are generated in a vehicle. Thus, the fusible gate resistor lacks robustness against noises.